The increasing utilization of microwave communication systems for the transmission of data has created a substantial need for a flexible wave guide exhibiting both low losses and uniform electrical characteristics in the millimeter microwave region. This need is especially acute in systems operating in the 20-150 GHz. range, where space considerations or mechanical limitations do not always permit the transmission of microwave energy from one item of equipment to another along a straight line path.
A number of flexible wave guides are currently being used in the microwave industry. One of the commonly used prior art flexible wave guides is the interlocking type which is made by spirally winding a strip of metal whose edge portions are folded and compressed during winding to form an interlocking structure. During flexure, the interlocking edges slide over each other, allowing the wave guide to assume the desired shape. This particular type of wave guide can be made with any desired cross-sectional shape, but is usually formed with a rectangular cross section insofar as the polarization characteristics of a wave guide with this cross section are the easiest with which to design. Although a wave guide of this kind works well at lower frequencies, its performance precipitously declines at higher frequencies, due to irregularities on the inner surface of the wave guide.
In an attempt to improve upon this type of flexible wave guide, some wave guides have been designed which include an interlock configuration which does not permit sliding. At the same time, U-shaped corrugations are provided between the interlocks. During flexure, these corrugations expand, compensating for the loss of flexibility caused by the inability of the interlock structure to slide. See, for example, U.S. Pat. No. 3,331,400. However, even this wave guide structure is unsuitable for use in the 20-150 GHz. range. The mechanical fabrication of a convoluted wave guide invariably results in a wave guide which includes irregularities which cause unacceptable attenuation in the millimeter microwave region due to the impossibility of manufacturing a wave guide having small enough convolutions to operate efficiently in the higher frequency microwave regions.
The inadequacies of these mechanically fabricated prior art wave guides are eliminated by the fabrication of a wave guide through the deposition of successive metallic layers upon a convoluted, chemically dissolvable arbor. Naturally, these metallic layers of the wave guide, which form the main section, conform to the shape of the arbor. Although multilayer plating processes have been used in the past, this technique has not been employed in the fabrication of wave guides which have extremely small convolutions. See, for example, U.S. Pat. No. 2,592,614.
Still another attempt to fabricate a high frequency microwave wave guide involves taking a non-grooved section of tubing and mechanically crimping it into an accordion-like shape. See U.S. Pat. No. 2,751,561. However, this type of device, like other mechanically manufactured convoluted wave guides, will not function in the millimeter region. This is due to the fact that it is not mechanically possible, using this techinque, to manufacture a wave guide with convolutions small enough to allow operation in the millimeter region. As one attempts to crimp the tubing with smaller and smaller convolutions, the convolutions become distorted and then become impossible to make.